Phototube and method of manufacture



June 11, 1946. R, 5, AME 2,401,736

PHOTOTUBE AND METHOD OF MANUFACTURE Filed Jan. 6, 1942 I VENTOR Haber-1' Janes.

ATTDRNEY Patented June 11, 1946 PHGTOTUBE AND METHOD OF MANU- FACTURE Robert B. .lanes, Verona, N. J assignor to Radio Corporation of America, a corporation of Dela- Ware Application January 6, 1942, SerialNo. 425,730

Claims. 1

My invention rel-ates to phototubes and particularly to phototubes having cathodes wherein the active cathode surface comprises antimony, arsenicor bismuth photo-sensitized with an alkali metal and to a new and improved method of manufacturing such phototubes.

Phototubes made in accordance with copending application Serial No. 379,010, filed February 14, 1941, of which I am co-applicant with Alan M, Glover, when designed for so-called end-on operation, that is, where a disc or cup-shaped cathode is provided opposite a phototube window with an intermediate ring-shaped or mesh anode, are somewhat unstable under different operating conditions, Such phototubes should become current saturated at a relatively low difference in potential impressed between the anode and cathode. However, such end-on tubes utilizing alkali metal sensitized antimony surfaces produce wide variations in cathode current with respect to the anode-cathode potential. Thus an increase in anode-cathode potential from 25 to 250 volts produces an increase in collected cathode current under uniform 1ighting conditions of from 50 to 150 percent of the current collected by the anode'at the lower potential. While a small variation in the collected cathode current, such as a variation of 10 percent may be tolerated for certainapplications in which such phototubes are used, a variation of from 50 to 150 percent based on the cathode current collected at the lower potential is too great for most applications. I have found that this variation in collected cathode current is inherent in end-on phototubes utilizing an alkali metal treated antimony photocathode, whereas such difficulties apparently are not encountered in tubes of the conventional silver-silver oxide-alkali metal type, such as the end-on type RCA 924. It is important, however, to provide an end-on phototube having high sensitivity in the blue portion ofthe spectrum because tubes of the RCA 924 type inherently have low blue sensitivity. Consequently, the problem of overcoming lack of stability in such tubes becomes of paramount importance.

It is an object of m invention to'provide an end-on phototube having high photosensitivity in the blue portions of the spectrum and which is stable under widely varying conditions of operation. It is also an object to provide an arsenic, antimony or bismuth-alkali metal type of phototube having high stabilityunder difierent operating potentials, It is also an object to provide a phptotube of thetype described suitable for unidirectional use which is stable and substantially unaffected by variation in potentials applied during operation, and it is a further object to provide a methodof manufacturing phototubes of the arsenic, bismuth or antimony alkali metal type which are inherently stable under diiferent potentials applied between the cathode and the anode.

In accordance with my invention I provide a phototube having a cathode of the arsenic, antimony or bismuth type sensitized with an alkali metal wherein the cathode is directly exposed to the envelope wall, the anode being so positioned that substantially all electrons liberated by the cathode during operation are collected by the anode irrespective of the potential applied between the cathode and anode. Further in accordance with my invention I provide a phototube and method of manufacture providing means to prevent collection of electrons liberated from the cathode by certain portions of the envelope wall thereby increasing the stability of the tube during operation.

These, and other objects, features, and advantages of my invention will at once become apparent from the following description thereof with reference to the accompanying drawing, wherein the single figure shows a phototube of the end-on type made in accordance with my invention.

l prefera'bly of cylindrical shape enclosing an electrically conducting foundation or cathode 2 which is preferably of concave disc or cup-shaped formation facing or exposed to the transparent end wall 3 of the envelope I, which serves as a window, so that light may be projected, through the end wall 3 and upon the cup-shaped surface of the cathode 2. The cathode 2 is preferably of metal coated on its inner or cup-shaped surface with a coating of antimony, arsenic or bismuth which is sensitized with an alkali metal such as caesium in accordance with my joint application referred to above. For example, a nickel cathode foundation may be coated with antimony or other metal, such as arsenic or bismuth, outside of the envelope l in accordance with my prior joint teaching, sealed within the cathode during operation are collected by the anode. More particularly in accordance with my invention I provide the anode concentric, that is, axially supported with respect to the cathode 2 and not closer than a minimum distance from the end wall 3 of the envelope, such that the electrons flowing along their normal trajectories from the cathode to the anode during operation are not intercepted by the end wall 3. The cathode and cooperating anode are therefore formed and positioned with respect to the envelope such that the electrostatic lines' of force between the cathode and anode, and along which the electrons flow, are not intercepted by the envelope wall. I have found as a result of a great number of tests on tubes made in accordance with my in vention, in comparison with tubes made prior to the completion of my invention, that the positioning of the anode 5 with respect to the cathode Z and end Wall '3 is some what critical and that if the anode 5 extends Within a critical distance of the envelope wall, the wall is interposed in the normal trajectory of the electrons and that as a consequence, the wall of the tube assumes a pe tential which'varies in accordance with the potential applied'between the cathode and anode. I have found that the anode must be in the form of .a' rod symmetrically positioned with respect to the disc or cup-shaped cathode and extending through the approximate center of the cathode. Thus, the use of any structural extensions, such as a ring 'or a mesh attached to the end of the anode adjacent the bulb wall, accentuates instability during operation. Especially is this true in a construction utilizing an anode lead extending along the envelope surmounted by caring anode as used in the silver oxide-caesium phototube type RCA 924, and such a construction will not achieve the objects of my invention in a tube utilizing an alkali metal sensitized antimony cathode as theinstability of such a tube renders the tube wholly unsuitable for use. Another example of an unsuitable construction is one in which the anode, while extending through the cathode, is provided with a wire mesh anode as disclosed by Geficken et al., 1,885,819. Such structures, while satisfactory when using alkali or alkaline earth metal sensitized cathodes, are wholly unsuited for use in combination with an alkali metal'sensitized antimony, arsenic, or bismuth cathode; Therefore. when I refer in the following description and in the claims to my anode as consisting of a rod-shaped member, I specifically exclude an anode surmounted by additional structure such as a ring or a wire mesh.

I prefer to maintain the length of the anode exposed to the photosensitive surface of the oath- Ode wholly within the rim of the cup-shaped cathode, that is, on the cathode side of a plane through the upper edge of the cathode, as shown in the drawing. Preferably the rod-type anode 5 as shown is mounted symmetrically with respect wherein the anode extended only from 5 to 8. mm., the collected cathode current variation was as small as 15 percent. In the preferred form, with the end of the anode below the rim of the cupshaped anode, the collected current variation may be reduced to less than 10 percent.

During the construction of a phototube made in accordance with my invention and as referred to above, the cathode 2 may be coated on its cupshaped surface with antimon or with a metal having similar properties, such as arsenic 0r bismuth, then treated following exhaustion of the envelope l with caesium or other alkali metal. However, in accordance with a further teaching of my invention, I provide a somewhat heavier coating of antimons or similar metal on the cathode 2 and vaporize a portion of this metal on the opposite end Wall to provide an inner antimony coating 6 of a predetermined thickness. During the sensitization of the antimony coating on the cathode the inner coating 5 on the end wall 3 is likewise sensitized with the alkali metal. Consequently any stra electrons which would normally impinge the endwall 3 strike the coating 6 liberating secondar electrons which are collected by the anode. Such liberation of secondary electrons causes the coating 6 to be maintained at a positive potential closely approaching that'of the anode.

In .manufacturing antimony, arsenic, or his- .muth cathodes sensitized with alkali metal I deto the cathode and extends through the approximate center of the cathode with its nearest approach to the end wall 3 not less than 2 mm. It is not a requirement, however, that the anode does not extend above the upper cathode edge, as tubes which I have made and in which anodes extended above the cathode by varying distances, such as 5, 10, and 12 mm. above the cathode but not less than 2 mm. from the end wall 3, have been satisfactory in that the variation in collected cathode current was only from 10 to 20 percent'when the anode voltage was changed from 25 to 250 volts, although in those tubes posit a relatively thick coating of antimony on a cathode foundation while the foundation is out-. side of the envelope. Preferably the deposition of antimony on the cathode foundation is by an evaporization and condensation. process per. formed in an evacuated envelope provided with antimony coated refractory filaments exposed to one or more foundation members so that the antimony may be directed to and condensed upon the member or members Following this formation of the antimony coating the foundation bearing the coating is sealed into an envelope as shown in the drawing with its coated side facing the end wall 3 and the envelope exhausted; No oxygenwhatsoever is introduced within the .envelope during the exhaust cycle because I have found that such introduction of oxygen substantially impairs the sensitivity of the completed tube. The'tube is then baked to remove occluded gases and in accordance with my invention is baked at sufiicient temperature to vaporize a portion of the antimony from the cathode allow ing the vaporized antimony to condense on the end wall 3. Thus, during the normal cooling following vaporization of a slight amount of antimony from the cathode, this vaporized antimony condenses on the end wall 3. During the sensitization of the cathode by an alkali metal obtained by flashing an alkali metal bearing compound, such as the activator 4, a portion of the alkali metal is deposited on the coating 6 and combines therewith probably forming an antimony-alkali metal alloy which has high secondary electron emitting properties, provided the antimony coating 6 is of the properthicknessr I, therefore, vaporize only sufficient antimony from the cathode 2 which when deposited on the end wall 3 and further When subjected to an alkali metal such as caesium, is of a light reddish color. This thickness corresponds to a thickness of 50 to angstroms, is substantially transparent, and

may be obtained by baking the'tube at 300 C. for V a period of approximately one-half hour. The

period of baking may vary for different tube constructions, but should be for such a length of time as to produce a reddish coating when treated with alkali metal. An end-on phototube made in accordance with this teaching of my invention has a minimum of collected cathode current variation with large variation in applied anode potential and consequently is very stable in operation. I have also found a coating of the specified color and thickness provides more stable operation in tubes wherein the anode is of conventional form such as a rod concentric with an arcuate shaped cathode.

While I have described my invention in connection with the use of antimony, it is to be understood that I do not wish to be limited to the use of this particular metal in forming a cathode or wall coating inasmuch as I have found arsenic and bismuth the full equivalents of antimony. Therefore, while I have indicated the preferred embodiments of my invention and a preferred method of manufacture, it will be apparent that my invention is by no means limited to the exact form illustrated or to the use indicated, but that many variations may be made in the particular structure without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. A phototube comprising an envelope having a window, a cathode member having an alkali metal sensitized coating of a metal selected from the group of metals consistin of antimony, arsenic and bismuth, an anode within said envelope and adjacent to said cathode, said anode being separated from said window by at least 2 mm., and a substantially transparent alkali metal sensitized coating of said metal on said window, said coating being of sufficient thickness to be reddish in color.

2. A phototube comprising an envelope, a cupshaped cathode foundation having its concave surface facing one wall of said envelope and bearing a coating of a metal selected from the group of metals consisting of antimony, arsenic and bismuth, said metal being sensitized by an alkali metal, and an anode extending through said cathode to within a distance not greater than 2 mm. from the wall of said envelope, said anode consisting of a rod-shaped member lying along the axis of said cup-shaped cathode.

3. A phototube comprising an envelope having a window, a cathode foundation having an alkali metal sensitized coating of a metal selected from the group of metals consisting of antimony, arsenic and bismuth facing said window, and an anode extending through said cathode and adapted to collect electrons therefrom, that portion of the anode which is exposed to the surface of said foundation bearing said coating consisting of a rod member extending in a direction toward said window but separated from said window by at least 2 mm.

4. A phototube as claimed in claim 3 including a coating of said metal selected from said group on the inner wall of said window oppositely disposed from the coating of said metal on said cathode.

5. A phototube comprising an evacuated envelope having a window, an anode and a cupshaped cathode facing said window and including an alkali metal sensitized metal selected from the group of metals consisting of antimony, ar- 7 senic and bismuth within said envelope, said anodebeing coaxial with the cathode and having its free end entirely within the concavity of said cup-shaped cathode. V

6. A phototube as claimed in claim 5 including a coating of the metal selected from said group of metals on the inner wall of said tube oppositely disposed from said cathode, said coating being of a reddish color.

7. A phototube comprising an evacuated envelope having a window, an alkali metal sensitized antimony coated cup-shaped cathode and an anode within said envelope, the concavity of said cathode facing saidwindow and said anode consisting of a rod coaxial with the cathode and havin an end entirely within the concavity thereof. 8. A phototube as claimed in claim '7 including a coating of antimony on the wall of said tube oppositely disposed from said cathode, said coating having a thickness not exceeding Vangstroms and being of a reddish color.

9. The method of manufacturing an end-on phototube comprising coating a foundation member with a coating of a metal selected from the group of metals consisting of antimony, arsenic and bismuth, positioning said coated foundation member in an envelope with the coating exposed to a portion of said envelope, supporting a cooperatin anode concentrically of said support member so as to extend to within said envelope a distance not less than 2 mm., vaporizing a portion of said metal coating allowing it to condense upon said envelope, and sensitizing said cathode and the condensed metal on said envelope with an alkali metal.

10. The method of manufacturing an end-on phototube comprising coating a cup-shaped cathode foundation with a coating of antimony, supporting a cooperating anode symmetrically through said cup-shaped cathode foundation, vaporizing sufficient antimony from said cathode foundation to provide a film of antimony on said envelope wall opposite said cup-shaped foundation of sufiicient thickness that when sensitized with alkali metal the metal on said wall is of a reddish color, and sensitizing said antimony with alkali metal.

ROBERT B. JANES. 

